WHAT MAKES IT RO­TATE?

Air & Space Smithsonian - - In The Museum -

AS THE GREAT SEPTEM­BER GALE of 1815 threw boats down the streets of Prov­i­dence, Rhode Is­land, John Far­rar watched the winds in Bos­ton swing around the com­pass from northeast to east to south in a mat­ter of a few hours. Records later showed that around the same time in New York, the wind was shift­ing, too, but in dif­fer­ent di­rec­tions. Far­rar de­duced that the whole storm must have been ro­tat­ing, like a gi­ant turntable.

Even now, 200 years later, this phe­nom­e­non (what me­te­o­rol­o­gists call a hur­ri­cane’s pri­mary cir­cu­la­tion) still looks like magic, be­cause there’s ab­so­lutely no force act­ing to spin the storm. And it is, in a sense, an il­lu­sion.

Be­cause Earth is a sphere, the “ground” at the equa­tor, where the planet is fat­ter, has far­ther to ro­tate in the same amount of time than the ground at, say, 40 de­grees lat­i­tude. The wind bound for the low-pres­sure cen­ter of a trop­i­cal storm can travel a long way, as far as 1,000 kilo­me­ters (620 miles). Over such a long dis­tance, the ro­ta­tion of Earth ap­pears to de­flect those winds from a straight line into a curve. Sci­en­tists and mariners had seen this ef­fect in the trade winds for cen­turies, but it wasn’t un­til the early 18th cen­tury that a me­te­o­rol­o­gist, Ge­orge Hadley, worked out why: The winds don’t just have for­ward mo­men­tum from the at­mo­spheric forces act­ing on them, they also have lat­eral mo­men­tum in the di­rec­tion Earth is ro­tat­ing.

So winds keep curv­ing—to the right in the north­ern hemi­sphere and to the left in the south­ern, un­til they’re ba­si­cally trav­el­ing in a cir­cle. It’s called the Co­ri­o­lis ef­fect, af­ter the French math­e­ma­ti­cian who later de­scribed it with an equa­tion. In the case of trop­i­cal cy­clones, the cir­cling winds are also still trav­el­ing in­ward, so the whole sys­tem now looks like a spi­ral. Be­cause of the cur­va­ture of the spi­ral, a par­cel of air mov­ing to­ward the cen­ter takes a longer route, so it trav­els over more warm sea­wa­ter and picks up more mois­ture and heat. This gives the storm more power, mak­ing it es­sen­tially self­sus­tain­ing. When sur­face winds reach 39 mph, me­te­o­rol­o­gists start call­ing the cy­clone a trop­i­cal storm and give it a first name. If the wind speed in­creases to 74 mph, ac­cord­ing to ad­mit­tedly ar­bi­trary guide­lines, it’s a hur­ri­cane.

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